fbdev/metronomefb: Use hardware device for dev_err()

Replace the use of the fbdev software device, stored in struct
fb_info.dev, with the hardware device from struct fb_info.device
in load_waveform(). The device is only used for printing errors
with dev_err().

This change aligns load_waveform() with the rest of the driver and
prepares fbdev for making struct fb_info.dev optional.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613110953.24176-18-tzimmermann@suse.de